The incorporation of ever-higher degrees of functionality into electronic systems, while making maximum use of available bandwidth in dense spectral environments, places stringent demands on filters that are tasked with the preservation of wanted signals and the suppression of unwanted ones. Filters and banks of filters in the form of so-called frequency multiplexers assume critical roles in many electronic systems, tasked with the suppression of unwanted signals that threaten to compromise system performance, while preserving wanted signals. The perennial challenge is to reduce unit size and production cost without undue sacrifice of filter performance. In addition to frequency selectivity, a filter's passband insertion loss normally constitutes one of the primary design concerns, be it to minimize noise in receiver front ends or signal attenuation in exciter applications. In the latter, thermal constraints may add to the design challenge.
Among the most compact and cost-effective filter solutions available are ones that rely on planar circuit topologies that employ constant-thickness layers of dielectric materials in conjunction with thin strip conductors for guiding propagating waves, exemplified by familiar implementation formats such as microstrip, stripline, and some versions of low-temperature cofired ceramic (LTCC). Among the principal drawbacks of these formats is elevated passband insertion loss that results from high current densities at the conductive strips' thin edges. Under resonant conditions, as encountered especially in bandpass filters, this invariably leads to high signal attenuation at passband frequencies and compromised frequency selectivity. A further concern may arise when dielectric layers of relatively poor thermal conductivity impede the extraction of loss-induced heat from the strip conductors, with power handling limited by heat-generated mechanical stresses. Similar concerns also apply, albeit to a lesser extent, to popular coaxial-type structures and other filter realizations that conceptually rely on two-conductor-based wave propagation with predominantly transverse electromagnetic fields.
In contrast, three-dimensional (3D) filter structures that are composed of coupled, metal-clad, dielectric-filled, single-conductor waveguide cavities, whose wave-guiding peripheries constitute single conducting envelopes, can distribute currents within the inner surfaces of these envelopes more optimally. This permits high current densities to be avoided, resulting in best-possible transmission-loss characteristics and frequency selectivity for a given aggregate filter volume. Furthermore, with electrical currents conducted exclusively in peripheral waveguide surfaces that are externally accessible and from which heat generated through dissipation can be easily extracted, these types of filters can handle very high levels of incident power. This results in filters with not only superior electrical performance, but also with excellent thermal performance for a given size.
Among the drawbacks of conventional 3D-waveguide filters are bandwidth limitations imposed by the practical need to operate in a regime where electromagnetic waves propagate only in a single mode. The limitations result from the absence of wave propagation below a geometry-determined cutoff frequency and the emergence of higher-order wave-propagation modes above a geometry-determined upper frequency limit. As an example, for common rectangular waveguide, the upper frequency bound is generally twice the low-end cutoff frequency, which imposes unacceptable constraints in cases where filters must cover multiple octaves. Furthermore, per-unit fabrication costs of 3D waveguide filters are generally higher than for contending planar-circuit counterparts.
The use of ridge waveguide is particularly attractive, as this allows considerably broader frequency coverage than conventional rectangular waveguide, relaxing bandwidth constraints while still retaining most of the advantages of 3D waveguides. Ridge-waveguide structures utilize capacitive loading in the cross-sectional centers of the guides to lower respective cutoff frequencies, while essentially not affecting upper frequency bounds, thereby increasing available percentage bandwidth, often by a substantial amount. As for the positioning of the lower and upper band limits on an absolute frequency scale, assuming application-predetermined maximum cross-sectional dimensions of the waveguide, this can be achieved by filling the internal regions of pertinent waveguide sections with a dielectric material of a suitable relative dielectric constant, whereby frequencies bounds simply scale proportional to the square root of the effective dielectric constant. Over the past twenty years, research has concentrated on exploiting the advantages of ridge waveguide and derivatives thereof for use in filters and frequency multiplexers that must cover wide frequency range. Current needs pertain, in particular, to the miniaturization of such devices.